python - 将数组从 np.triu_indices 转换为对称矩阵

Question

我可以使用以下方法轻松地将对称矩阵转换为其三角形分量之一的数组/一维矩阵

A_symmetric = np.matrix([[1,2][2,3]])
A_array = A_symmetric[np.triu_indices(2)] == np.matrix([1,2,3])

（与 类似tril_indices）

但是我怎样才能反转这个操作，即如何从给出的数组/一维矩阵中得到一个对称矩阵triu_indices？

Answer 1

如果你对数组而不是矩阵没问题，你可以简单地预先分配和分配：

>>> i, j = np.triu_indices(8)                                                                                       
>>> a = np.arange(i.size)                                                                                          

>>> M = np.empty((8, 8), a.dtype)
>>> M[i, j] = a                                                                                                     
>>> M[j, i] = a
>>> M                                                                                                               
array([[ 0,  1,  2,  3,  4,  5,  6,  7],                                                                            
       [ 1,  8,  9, 10, 11, 12, 13, 14],
       [ 2,  9, 15, 16, 17, 18, 19, 20],
       [ 3, 10, 16, 21, 22, 23, 24, 25],
       [ 4, 11, 17, 22, 26, 27, 28, 29],
       [ 5, 12, 18, 23, 27, 30, 31, 32],
       [ 6, 13, 19, 24, 28, 31, 33, 34],
       [ 7, 14, 20, 25, 29, 32, 34, 35]])